1. Field of Invention
The present invention relates to an electroluminescent device and a fabrication method thereof, and more particularly to an electroluminescent device and a fabrication method thereof with high efficiency.
2. Related Art
The light emitting diode (LED) is a cold light emitting element, which releases lights when energy released after electrons and holes are combined in a semiconductor material. According to different used materials, various monochromatic lights with different wavelengths are outputted. The LEDs may be mainly classified into a visible light LED and an invisible light (infrared) LED. Compared with the conventional lighting manner of a light bulb or lamp, the LED has advantages of power save, vibration resist, and high flicker speed, so that the LED has become an indispensable and important element in the daily life.
With reference to FIG. 1, a conventional LED device 1 is made by at least one LED element 10 attached to a transparent substrate 11. The LED element 10 includes a n-type semiconductor layer 101, a light emitting layer 102 and a p-type semiconductor layer 103 formed in sequence. A first contact electrode 104 is connected with the n-type semiconductor layer 101. A second contact electrode 105 is connected with the p-type semiconductor layer 103. When voltages are respectively applied to the semiconductor layers 101 and 103 to generate currents, the electrons and the holes of the n-type semiconductor layer 101 and the p-type semiconductor layer 103 are combined together so that electric power is converted into optical energy. As shown in FIG. 1, the LED element 10 is attached to the transparent substrate 11 by a transparent adhering layer 12. To raise the current distribution efficiency, the junction surface between the LED element 10 and the transparent adhering layer 12 further uses a transparent conduction layer 13. The overall brightness of the LED device 1 is increased by evenly distributing the current.
As shown in FIG. 1, the first contact electrode 104 is formed on the n-type semiconductor layer 101. The second contact electrode 105 is formed on the transparent conduction layer 13. That is, the first contact electrode 104 and the second contact electrode 105 are disposed at the same side of the transparent substrate 11. Therefore, the production of the LED device must involve the process of removing part of the LED element 10 by etching, for example, the area A in FIG. 1 is used for disposition of the second contact electrode 105. However, in addition to increase the complexity of the production process, this step may cause a leakage current of the LED element 10 due to its uneven surface as a result of the bad control in etching. This will lower the yield of the LED device 1.
In the prior art it is common to utilize the epitaxial substrate as the transparent substrate and utilize an organic adhering material to form the transparent adhering layer. Since the epitaxial substrate and the organic adhering material have low thermal conductivity coefficients, they cannot provide a better heat dissipation path for the LED element 10. As a result, the heat generated from the operating LED device 1 accumulates and affects the light emitting efficiency thereof.
Current researches in the LED put emphases on how to extract the photons generated from the LED element 10 in order to reduce the unnecessary heat caused by repeated reflections and absorptions of the photons therein. It is thus important to lower the operating temperature of the LED device 1 in order to dissipate heat generated from the LED element 10.